As lesbian relationship-as-catalyst movies go, Showtime's A Girl Thing (2001) is one of the better ones.
The movie is actually four separate but interrelated tales revolving around patients of psychiatrist Dr. Beth Noonan (Stockard Channing). While all four are centered on women and their relationships with one another, the first story is the only one that deals explicitly with a sexual relationship between women.
Sub-titled "It Don't Mean a Thing If It Ain't Got That Swing," this story explores the experience of Elle Macpherson's Lauren, a relationship-phobic lawyer who doesn't know how to reconcile her attraction to a bisexual woman, Casey (Kate Capshaw), with her heterosexual identity.
The movie intersperses Lauren's conversations with Dr. Noonan with flashbacks of her relationship with Casey as it unfolds: The two women meet on a blind double-date and instead of hitting it off with the men they've both been set up with, they find themselves drawn to each other.
Lauren is extremely confused by her attraction to Casey, and tries to process it with Dr. Noonan and with her friend Claire at the office. To Lauren's surprise, Claire reacts very negatively and reveals homophobia that neither woman knew she had, and Lauren's affair with Casey is soon the subject of office gossip. Lauren stands up to Claire, and Claire eventually apologizes and admits she just reacted negatively because she was scared, but not before she has reinforced all of Lauren's worst fears about being rejected.
Casey's straight friend, on the other hand, is very supportive and even eager to get all the details of Casey's relationship with Lauren.
Lauren and Casey bumble through their first date, their first kiss, and finally, sex, all the while Lauren is protesting to herself and everyone else that she's not a lesbian. She exhibits all the behavior of a freaked-out previously-straight woman in her first lesbian relationship: she drinks too much on their first date to bolster her confidence, she won't hold Casey's hand in public (except in the lesbian dance club), and although she clearly enjoys sex with Casey, she is never the aggressor sexually.
Ultimately, Lauren breaks off the relationship with Casey because she just can't "go against the grain." Dr. Noonan points out that her Lauren's problem may be that she isn't capable of a relationship with anyone, that it isn't that Casey is a woman that is the problem, but that Lauren can't control Casey.
The fact that Casey is so "absolutely comfortable in the world with herself" is too difficult for Lauren to handle since Lauren is so decidedly uncomfortable with herself.
But even after she breaks it off, Lauren expresses happiness that she "did something daring" for once in her life, or as Dr. Noonan states, she "led with her heart." Dr. Noonan's private assessment of the situation is that Lauren "experienced something joyous, but then had it turned into something dirty by the judgment of the outside world, and by her own judgment of herself."